You can use number_format like:
$n = 2.1;
echo number_format($n, 2, ','); // 2,10
If you have commas as decimal separators in your input you can convert values to float with:
$number = floatval(str_replace(',', '.', str_replace('.', '', $string_number)));
str_replace('.', '', $string_number)
is used to remove thousand separators.
str_replace(',', '.', ... )
is used to replace commas with dots.